The present invention relates to hoisting machines such as rope hoists or chain blocks and, more particularly, to such hoisting machines provided with an overload protection apparatus.
Conventional overload protection apparatus, for example such one as used in a chain block merely for the purpose of avoiding an overload operation at the time of commencement of hoisting, consists of a slip gear which is incorporated in a reduction gear train and adapted to perform a power transmission by means of friction.
Namely, in the overload protection device of this type, any one gear in a reduction gear train on a sprocket shaft is split into two halves between which interposed is a lining. The torque transmission is performed, through the lining, as one of the halves is resiliently pressed on to the other by a resilient member such as a disc spring or the like, the pressing force being adjustable by means of, for example, a screw.
This overload protection apparatus has, however, a fundamental drawback that the load once lifted up may accidentally be lowered, due to an unexpected application of an external force caused by vibration, swinging or the like, when the load torque is large and is scarcely withstood by the resistance torque provided by the slip gear.
Thus, when a hoisting machine incorporating this conventional overload protecting device is used as a lift, the load once lifted up and now suspended may dangerously fall down when an excessive load torque is applied to the hoist.
At the same time, when the height of the lifted load is adjusted by so-called inching operation which consists in winding and unwinding the rope repeatedly, an impact force may be generated to break the balance of the torque resulting in an accidental lowering of the load.
In any way, this conventional technique relies upon an unstable frictional force, and this conventional apparatus may act against the will to incur a danger, although it is inherently designed as a safety device.